Britain will respond "robustly" if any foreign government was behind the sudden and severe illness of a former Russian spy and his daughter, Foreign Secretary Boris Johnson has said, warning that no attempt to take a life on U.K. soil will go unpunished.

Speaking to Parliament on March 6, Johnson suggested that Britain could step up sanctions against Russia and reconsider its participation in the soccer World Cup this summer if Moscow was involved in the incident in the southern English city of Salisbury.

"We don't know exactly what has taken place in Salisbury, but if it's as bad as it looks, it is another crime in the litany of crimes that we can lay at Russia's door," Johnson said.

Sergei Skripal and his daughter, Yulia Skripal, remained in critical condition two days after being found unconscious on a bench outside a shopping mall in Salisbury. Police suspect they were exposed to an substance that caused their illness.

"Should evidence emerge that implies state responsibility, then Her Majesty's government will respond appropriately and robustly," Johnson told Parliament.

"I am not now pointing fingers," he said. "I say to governments around the world that no attempt to take innocent life on U.K. soil will go either unsanctioned or unpunished."

He indicated that strong evidence of Russian government involvement could lead to new punitive measures against Moscow, which is under European Union and U.S. sanctions over its aggression in Ukraine and other actions.

"If the suspicions...prove to be well-founded, then it may very well be that we are forced to look again at our sanctions regime and other measures that we may seek to put in place," Johnson said.

"It is clear that Russia, I'm afraid, is now in many respects a malign and disruptive force, and the U.K. is in the lead across the world in trying to counteract that activity," he said.

Johnson also said that if Russia was behind the incident, "I think it would be very difficult to imagine that U.K. representation at [the World Cup] could go ahead in the normal way."

Reuters later quoted a British government source as saying that Johnson was referring to the attendance of official representatives and did not intend to suggest that England's soccer team could be pulled from the June 14-July 15 tournament in Russia.

The unexplained incident in Salisbury swiftly drew comparisons with the death of Aleksandr Litvinenko, a former Russian security agent who fell ill and died in London in November 2006 after ingesting radioactive polonium-210.

A British inquiry concluded that the Russian government was behind Litvinenko's death and that President Vladimir Putin "probably approved" the killing.

Johnson confirmed media reports that had identified the man and woman found unconscious in Salisbury on March 4 as Sergei Skripal, 66, and his daughter Yulia, 33.

Mark Rowley, Britain's top counterterrorism officer said that police investigating the incident were "alive to the fact of state threats."

"We're speaking to witnesses. We're taking forensic samples at the scene. We're doing toxicology work and that will help us to get to an answer," Rowley said.

Skripal is a former colonel in Russia's GRU military intelligence agency who was convicted of passing state secrets to Britain in 2006 but was released from prison -- and sent to the West -- in a spy swap in 2010.

Putin's spokesman said that Russia had "no information" on what could have led to the incident, which he called a "tragic situation," and indicated that Moscow was ready to cooperate with British authorities if asked.

"Moscow is always open to interaction," Kremlin spokesman Dmitry Peskov said. He said that Russia had not received such a request.

Asked to respond to British media speculation that Russia had poisoned Skripal, Peskov said, "It didn't take them long."

The Russian Embassy in London said the incident was being used to demonize Russia.

Russia's Foreign Intelligence Service (SVR) said it had no comment to make, Reuters reported. There was no immediate comment from the Foreign Ministry or the Federal Security Service (FSB).

While Skripal appeared to be one of several Russians who have suddenly become ill or died in Britain in recent years, British authorities said that nobody should assume his illness was the result of foul play.

The authorities "are conducting some extensive inquiries to determine exactly what led to these two people falling unconscious and clarify whether or not any criminal activity has happened," Wiltshire police official Craig Holden said.

Parts of the Salisbury center remained sealed off on March 6 as emergency responders in hazardous-material suits continued to canvas there.

Police vehicles were also seen at what was listed as Skripal's home in Salisbury.

The National Health Service said it had only limited information about the patients, but there "doesn't appear to be any further immediate risk to public health."

The substance has not been identified. But local media reported that emergency services suspect the powerful synthetic opiate fentanyl may have been involved.

"They looked like they'd been taking something quite strong," the BBC quoted an eyewitness as saying.

A still image taken from an undated video shows Skripal being detained by secret service officers in an unknown location.

"She was sort of leant in on him. It looked like she had passed out maybe," the eyewitness said. "He was doing some strange hand movements, looking up to the sky."

Skripal was arrested in Moscow in December 2004 and convicted by a Moscow military court in August 2006 of "high treason in the form of espionage."

He was found guilty of passing the identities of Russian intelligence agents working undercover in Europe to Britain’s Secret Intelligence Service, or MI6, in return for $100,000.

Russia’s FSB alleged he had begun working for MI6 while serving in the army in the 1990s.

The death of Litvinenko, who moved to Britain and had become a vocal critic of Putin, severely strained relations between London and Moscow.

British police stand guard beside a cordoned-off area in Salisbury.

Coming weeks after investigative journalist Anna Politkovskaya was shot dead in Moscow, it deepened concerns about the risks run by Russians who challenge the Kremlin -- wherever they live.

In findings issued in January 2016, British investigators said there was a "strong probability" that Litvinenko's poisoning was carried out by Russians Andrei Lugovoi and Dmitry Kovtun, acting under orders from the FSB.

They concluded that Litvinenko ingested polonium-210 while drinking tea in a luxury London hotel with Lugovoi and Kovtun. He died in a London hospital three weeks later, on November 23, 2006.

Russia has dismissed the inquiry as "opaque" and "politically motivated" and has refused to extradite the suspects.

Kovtun and Lugovoi, who is now a deputy in the Russian parliament, have denied involvement despite traces of polonium that British investigators say the two left across London.

Igor Sutyagin, a Russian arms-control researcher who was also part of the exchange of spies in 2010 and lives in Britain, told the online news organization RTVI that he only knew Skripal briefly, during the time they were being flown from Moscow to London.

He said the incident might have been insidious but appeared to downplay that possibility, suggesting that Russia might not want to risk a further blow to its reputation in the West.

"Such events are not really in Russia's interests right now," Sutyagin said. "The Kremlin has so many problems falling on its head these days, one more would be too much."

But in a reference to the killing of opposition politician Boris Nemtsov, who was shot dead near the Kremlin in Febuary 2015, he said that "Nemtsov set the pattern and now [the Russian authorities] have this reputation.”

With reporting by the BBC, Reuters, AP, AFP, dpa, and Press Association